The cell-surface protein APO-1 is a member of the nerve growth factor (NGF)/tumor necrosis factor (TNF) receptor superfamily. APO-1 mediates apoptosis in susceptible cells upon stimulation with the monoclonal antibody anti-APO-1 or upon binding of its natural ligand. Soluble receptors had previously been identified for most members of the NGF/TNF receptor superfamily. Recently, a soluble form of APO-1 (sAPO-1) was described. We established a sandwich enzyme-linked immunosorbent assay to detect sAPO-1 in culture supernatants of human cell lines and in human sera. sAPO-1 was found in culture supernatants of different human B- and T-cell lines. Molecular weights of sAPO-1 and membrane APO-1 were similar. In addition, in comparison to healthy donors, sera from patients with different high- and low-grade malignant B- and T-cell leukemias and lymphomas contained increased levels of sAPO-1. These findings may have implications for the growth of leukemias and the diagnostic monitoring of individual patients.